Since aerospace structures always pursue the optimal performance, they have become the frontier to test and improve advanced composite material technology. Raising the amount of composite material is crucial to the development of lightweight and high-performance aerospace equipment. Restricted by traditional manufacturing technology, the fiber placement path of fiber reinforced polymer laminates is always parallel straight, and orientation angles of the same layer of the fibers are fixed, such as 0°, 90° and ±45° always used in the project, which narrows the design space of fiber reinforced polymer laminates. By comparison, in variable stiffness design, the placement paths of the same layer of the fibers are continuous curve. According to requirement of structural-load-carrying capacity, the fiber path can adjust the local strength and stiffness to change the distribution of stress in a layer plane, so as to effectively improve structural integrated performance (such as buckling load and breaking strength). The variable stiffness design has been applied into the design of foreign aircraft fuselage structure and special-section wing structure. NASA believes that the structural elastic tailoring is a key technology to manufacture low-cost and lightweight subsonic aircraft in the future, that is, designers can use a high-strength and stability-constrained material to make the stiffness characteristic of load-carrying structure distributed as needed. In recent days, increasingly rigorous design requirements of large diameter, thin-walled and ultra-lightweight heavy rockets and large aircrafts in China, and the trend of multi-function fusion of load-carrying structures lead to growing cut-outs, complicated inner load path and stress distribution, and remarkably increased exposed risk of structural uncertainty. Obviously, for aaerospace spacecabinshell with complex load path, the promising variable stiffness composite plate and shell structure has great appeal.
The load capacity of fiber reinforced polymer structures is influenced by material property, manufacturing tolerance, load variation and other uncertainty factors. Moreover, since variable stiffness structures greatly extend the design space, the deviation of spatial distribution position of fiber orientation angles caused by the manufacturing process becomes a new uncertainty factor, which significantly increases the risk which is caused by structural uncertainty. Thus, it is urgent to apply reliability-based design optimization into variable stiffness composite structures. In order to improve the efficiency of optimization, isogeometric analysis has been utilized to the shape optimization design. The design model and analysis model have adopted the same geometric description in the isogeometric analysis, which can easily analyze the sensitivity directly and abandon the accuracy error caused by mesh approximation in calculating the traditional finite element sensitivity. It can be predicted that compared with the traditional finite element method, the optimization design of variable stiffness plate and shell structures based on isogeometric analysis can dramatically shorten the calculation time and satisfy the higher precision requirement.
The existing traditional reliability-based design optimization of variable stiffness composite plate and shell structures needs heavy computational cost and excessively depends on the finite element analysis. The reliability-based design optimization needs to call fine finite element analysis frequently. However, its optimization efficiency is extremely low even if the isogeometric analysis technology is employed. Beyond that, the inaccurate first-order reliability method is widely used in engineering due to the limitation of computing resource, to conduct reliability optimization. Nevertheless, for the aerospace field which demands rigorous reliability, reliability analysis with high efficiency and high accuracy is rather vital and imperative. Although plenty of researches have been carried out in this field, an efficient and accurate integration method for accurate modeling and reliability optimization of variable stiffness composite plate and shell structures has not yet been given.